Secondary (rechargeable) lithium ion batteries are widely utilized in consumer electronic devices such as cell phones and laptop computers owing, in part, to their high energy density. A secondary battery stores electrical energy as chemical energy in two electrodes, an anode (the reductant) and a cathode (the oxidant). In a secondary rechargeable lithium ion battery, the anode and the cathode are kept apart inside the battery by a separator that is permeable to a lithium-ion electrolyte that allows lithium ions (Li+) to pass between the electrodes inside the battery, but forces the electrons to move in an external electronic circuit. The anode and the cathode normally include compounds into which lithium ions may be reversibly inserted. The electrolyte typically contains as lithium salt dissolved in an organic liquid to produce lithium ions. Often the electrolyte contains a flammable organic liquid carbonate. Conventional lithium ion batteries generally use an anode that has an electrochemical potential poorly matched to the energy at which the electrolyte is reduced, which results in a lower capacity and may introduce an internal short-circuit that sets the electrolyte on fire unless charging rates are controlled.
Conventional lithium-ion secondary batteries are designed so that the electrolyte has a window between its LUMO (lowest occupied molecular orbital) and HOMO (highest occupied molecular orbital). This window is typically between 1.1 and 4.3 eV below the Fermi energy (electrochemical potential) of elemental Lithium. Conventional lithium-ion secondary batteries also have an open-circuit voltage described by the equation:VOC=(EFA−EFC)/e where EFA is the Fermi energy of the anode, EFC is the Fermi energy of the cathode, and e is the magnitude of the charge of an electron. If EFA lies above the energy of the electrolyte's LUMO, the electrolyte will be reduced during use of the battery unless a passivation layer forms on the anode surface. Such a solid-electrolyte interphase (SEI) passivation layer contains elemental Lithium (Li0) in order not to block lithium ion transfer across it.
When a conventional lithium ion secondary battery is charged, lithium ions are transferred from the electrolyte to the anode. Electrons (e−) are also transferred to the anode at the same time. Higher voltages can be used to charge batteries more quickly, but if the voltage used in order to obtain a fast charge raises the energy of the incoming electrons above the Fermi energy (electrochemical potential) of metallic lithium, the lithium ions will inhomogenously plate out of the electrolyte onto the anode as elemental Lithium. If such a process occurs, the anode can develop a mossy surface and, eventually, a lithium dendrite can grow through the electrolyte to the cathode and short-circuit the battery with catastrophic results, such as a fire.
To prevent such short-circuits, carbon is typically used as the anode material into which lithium ions are be reversibly inserted. Insertion of lithium ions into carbon is a two phase reaction from C to LiC6 and provides a flat voltage of approximately 0.2 V versus Li+/Li0. Unfortunately, the electrochemical potential of reduced carbon is above the electrolyte LUMO and thus carbon anodes form a passivating SEI layer as described above. This layer increases the impedance of the anode, robs lithium irreversibly from the cathode on the initial charge, and limits the charging voltage and thus the rate of charge. If the cell is charged too rapidly (typically at a voltage of 1.0 V versus Li+/Li0), lithium ions are not able to traverse the SEI layer before they are plated out on the surface of the SEI layer as elemental Lithium, also as described above. This problem limits the rate of charge of a battery and can necessitate additional circuitry as a safety measure against battery short-circuits. In addition, the capacity of the cathode normally limits the capacity of a cell, and the entrapment of lithium in the anode SEI layer during charge can reduce the capacity of the cathode and therefore the energy stored in the cell.
One alternative anode material is the spinel Li4Ti5O12 (Li[Li1/3Ti5/3]O4). Such an anode operates on the Ti(IV)/Ti(III) redox couple located at 1.5 V versus Li+/Li0. Such anodes are capable of a fast charge and a long cycle life because no SEI layer is formed. However, the material has a low specific capacity (≈120 mAh/g) and the loss of 1.3 V relative to a carbon anode reduces the relative energy density of a battery using such a titanium-based anode. Therefore, there is a motivation to identify a solid anode material with a higher capacity and having a voltage in the range of 1.1≦V≦1.5 V versus Li+/Li0.